This invention is generally directed to processes for the preparation of colloidal dispersions of chalcogenide alloys, and more specifically the present invention is directed to improved processes for preparing dispersions of selenium, tellurium, and their alloys. One general embodiment of the present invention is directed to improved processes for the preparation of selenium, tellurium, and selenium tellurium alloys by adding a source composition with the aforementioned substances therein to a solution comprised of a polymeric material dissolved in an organic solvent, and subsequently subjecting this solution to a reduction reaction. Compositions prepared in accordance with the process of the present invention are useful for a number of purposes, including for example as photoconductive substances in xerographic imaging processes. Accordingly, the colloidal dispersions of selenium tellurium alloys prepared in accordance with the process of the present invention can function as photogenerating pigments in layered imaging members, inclusive of those described in U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference.
Processes for the preparation of selenium and selenium alloys in high purity, 99.999 percent or greater, thus enabling their selection for incorporation into xerographic imaging members are known. These processes, however, involve a number of chemical and physical processing steps, and additionally require high temperature distillations. Accordingly, therefore, many of the prior art processes for preparing selenium, or selenium tellurium alloys of high purity are complex; and in many instances economically unattractive. Specifically, the chemical and physical prior art processes can result in the formation of hazardous volatile oxides during the high temperature distillation sequence. Furthermore, the aforementioned prior art processes can generate selenium, or selenium alloy products of different electrical properties despite adherence to the same process conditions.
One present specific common commercial method selected for the preparation of high purity selenium, or the corresponding alloys thereof, encompasses the formation of selenious acid, H.sub.2 SeO.sub.3, from crude selenium, followed by purification, and a complicated and repeated ion-exchange process. The selenium precipitate is then further purified, melted, and subjected to distillation at relatively high temperatures of from about 600.degree. C. to 700.degree. C., followed by vacuum distillation. This distillation necessitates very complex and costly equipment; and further, any pollution products resulting such as vaporous oxides and mercury must be safely eliminated.
There is disclosed in U.S. Pat. Nos. 4,007,255 and 4,009,249 the preparation of stable red amorphous selenium containing thallium, and the preparation of red amorphous selenium. In the U.S. Pat. No. 4,007,225 there is illustrated a process for generating an amorphous red selenium with thallium, which comprises precipitating selenious acid containing from about 10 parts per million to about 10,000 parts per million of thallium dioxide with hydrazine from a solution thereof in methanol or ethanol containing not more than about 50 percent by weight of water at a temperature between about -20.degree. C. and the freezing point of the solution; and maintaining the resulting precipitate at a temperature of about -13.degree. C. to about -3.degree. C. until the solution turns to a red color. The U.S. Pat. No. 4.009,249 presents a similar disclosure with the exception that the thallium is not contained in the material being treated.
Additionally, a number of other processes for the preparation of selenium and selenium alloys are known. Thus, for example, there is disclosed in U.S. Pat. No. 4,121,981 an electrochemical method for obtaining a photoreceptor comprised of a selenium tellurium layer. More specifically, there is described in this patent the formation of a photogenerating layer by electrochemically codepositing selenium and tellurium onto a substrate from a solution of their ions in such a manner that the relative amounts of selenium and tellurium which are deposited are controlled by their relative concentrations in the electrolyte, and by the choice of electrochemical conditions.
Moreover, there is disclosed in copending applications improved processes for affecting the preparation of selenium, and selenium alloys in high purity which overcomes many of the prior art disadvantages. These processes comprise the formation, isolation and purification of chalcogenide esters, followed by the reduction of these esters to obtain the compositions desired. There is thus described, for example, in U.S. Ser. No. 404,259, U.S. Pat. No. 4,548,800, the disclosure of which is totally incorporated herein by reference, a process for the preparation of selenium of high purity which comprises reacting selenious acid, selenium oxides, or mixtures thereof with an alcohol, followed by subjecting the resulting isolated selenium ester to a reduction reaction. In one variation of the process described in the copending application, the selenious acid, selenium oxides, or mixtures thereof are prepared by the reaction of crude selenium with a strong acid, such as nitric acid, sulfuric acid or mixtures thereof. This process is economically attractive since high temperature distillations and costly equipment are not required, and most of the processing materials can be recycled.
Furthermore, there is disclosed in U.S. Pat. No. 4,122,030 a method for the preparation of stable dispersions of selenium wherein there is provided a solution of a solvent containing submicron domains of a soluble second phase with a plurality of pendent groups capable of reducing selenious acid to selenium, followed by adding selenious acid or an alkyl selenite to the solution to reduce the selenium in the selenious acid or alkyl selenite to zero valent selenium, thereby resulting in the formation of a stable colloidal dispersion of selenium, reference the disclosure in column 2, beginning at line 5. Disadvantages associated with the process described in the U.S. Pat. No. 4,122,030 include the necessity of forming submicron domains capable of binding the reducing agents therein, and further the domains that are regenerated on completion of the reaction remain in the dispersions. As these domains are charged species, they have a tendency to trap carriers or positive charges, which is undesirable, when imaging members comprised of these compositions are selected for incorporation into layered photoresponsive devices. Moreover, selenium prepared in accordance with the process of this patent is red in color, that is, it is amorphous, while with the process of the present invention these are prepared crystalline selenium tellurium substances which are highly useful for incorporation into layered photoresponsive imaging members.
While the above processes are suitable for their intended purposes, there remains a need for improved processes for the preparation of certain chalcogens and chalcogenide alloys. Also, there remains a need for improved simple low temperature chemical processes for preparing selenium, tellurium, and selenium alloys in high purity, wherein the desired particles are generated in solution; and subsequently the solution is coated on a suitable substrate thereby resulting in the formation of a photoconductive member. There also continues to be a need for improved processes for obtaining chalcogenide particles in a polymeric solution. Additionally, there continues to be a need for improved processes for preparing selenium tellurium alloys in high yields, which eliminate environmental hazards associated with the formation and removal of harmful materials. Moreover, there continues to be a need for processes wherein photoconductive selenium tellurium substances are directly obtained from a polymer/solvent mixture.